Magnetic recording disk drives store digital data in the magnetic recording medium of a rotating storage disk. The disks can be coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at high speed. Data is stored as magnetic transitions on the disks in a plurality of concentric circular data tracks. The data is recorded and read by a read/write transducer or head. Each read/write head is located on the trailing end of an air-bearing slider, with the air-bearing surface (ABS) of the slider supporting the slider above the rotating disk. Each slider is mounted on the end of an arm of a rotary actuator that moves the sliders and attached heads across the disk surfaces. The write head can be an inductive write head with the magnetic elements being the magnetic poles that generate the magnetic write fields. The read head can be a magnetoresistive (MR) read head, which may be a spin-valve type giant magnetoresistance (GMR) sensing element, or the more recently proposed magnetic tunnel junction (MTJ) sensing element.
An important parameter affecting the recording density of a disk drive is the magnetic spacing between the magnetic elements (the write poles and the read element) and the magnetizable medium of the disk. Closer physical spacing (leading to closer magnetic spacing) allows for smaller magnetic “bits”, which in turn allow for narrower track widths and consequently a greater recording density.
The magnetic spacing on a storage disk can be limited to a certain extent by the thickness of a protective overcoat that is applied on the magnetic elements to inhibit corrosion. The spacing can also be limited by how far the read head elements are recessed from the air bearing surface of the slider, and by tolerances that must be built into the flying height of the slider to accommodate manufacturing and assembly.
A self-limiting wear contact pad (or “protrusion pad”) on an air bearing slider has been introduced to reduce the limitations noted above by reducing the built-in tolerances associated with slider fabrication and disk drive mechanical assembly, and by eliminating head overcoats and some head recession. In protrusion pad disk drives, the heads of the disk drive are encapsulated in a pad that protrudes towards the disk beyond the air bearing surface of the rest of the slider. The protrusion pad is designed so that it initially contacts the disk. The contact force between the pad and the disk depends on the extent of protrusion of the pad. The head overcoat and rest of the protrusion pad are relatively quickly “burnished” or worn away during contact with the rotating disk. The wear stops when the head no more contacts the disk with sufficient contact force. Thus, the wear is self limiting. The amount of pad to be burnished is designed such that the protective overcoat on the pad and most of the recession of the magnetic elements is eliminated. This leaves only a very small distance between the head and disk across the air bearing surface and, thus, permitting the use of smaller magnetic bits and, hence, denser recording. Protrusion pads are described in the present assignee's pending U.S. patent application entitled “Self-Limiting Wear Contact Pad Slider and Method for Making Same”, Ser. No. 09/681,234 filed Oct. 13, 2000, which is incorporated herein by reference. Additionally, to alleviate the risk of corrosion that arises when the head overcoat is worn away, vapor phase corrosion inhibitors (VPCI) to prevent corrosion of uncoated heads have been proposed in the present assignee's pending U.S. patent application entitled “Magnetic Recording Device with Improved Reliability”, Ser. No. 10/051,866 filed Jan. 15, 2002, which is incorporated herein by reference. As an alternative to VPCI, the present assignee's co-pending U.S. patent application Ser. No. 10/212,575, filed Aug. 2, 2002 and incorporated herein by reference, sets forth a way to hermetically seal a disk drive.
The present invention makes the critical observation that while protrusion pad-based heads are indeed effective in achieving greater data densities, it happens that during writing, current through the coil and the fluctuating magnetic flux through the yoke of the write element produces heat, causing the materials in the heated region to expand and protrude toward the disk. Although the protruding portions wear away in accordance with protrusion pad principles discussed above, subsequent to burnishing and when no write current is applied, the materials cool and retract away from the disk. Because the read head is mounted on the same protrusion pad as the write poles, it, too, retracts away from the disk during such periods, increasing the head-to-disk spacing during read operation and, thus, limiting the data density that can be achieved.
The present invention further understands that simply increasing the distance between the read head and write poles on the protrusion pad is less than optimum, because it requires a larger protrusion pad that burnishes more slowly than a smaller pad, owing to its size, and furthermore that can result in unintended lift of the head away from the disk, again owing to a relatively large surface area. This lift can impose undue constraints on the air bearing design. Having made the above critical observations, the invention disclosed herein has been provided.